1. Field of the Invention
This invention relates to tandem axle assemblies and, in particular, to an arrangement of gears within a tandem axle assembly.
2. Disclosure of Related Art
A conventional tandem axle assembly includes forward and rear axle assemblies and an intermediate drive shaft assembly connecting the two axle assemblies. The forward and rear axle assemblies each include a pair of axle half shafts extending therefrom on which one or more wheels of a vehicle are mounted. Each of the forward and rear axle assemblies further includes a differential gear set that allows the vehicle wheels on each axle assembly to rotate at different speeds. Each of the differential gear sets includes a pinion gear in mesh with a ring gear (which in turn drives a plurality of bevel gears to cause rotation of the axle half shafts). The pinion gears of the forward and rear axle assemblies are driven by an inter-axle differential housed within the forward axle assembly (with the rear axle assembly being driven by the inter-axle differential through the intermediate drive shaft assembly).
In many conventional tandem axle assemblies, the pinion gears and ring gears comprise hypoid gears and the axis of rotation for each pinion gear is offset from the axis of rotation of a corresponding ring gear (i.e., the two axes lie in different parallel planes). This offset improves contact ratio between the pinion gears and ring gears thereby resulting in stronger and quieter gearing. In these conventional tandem axle assemblies, the offset between the pinion gear and ring gear of the forward axle assembly is identical to the offset between the pinion gear and ring gear of the rear axle assembly.
Many of the above-described conventional tandem axle assemblies suffer from a significant drawback. In particular, the intermediate drive shaft assembly includes an output yoke and an input yoke that exit and enter, respectively, the forward and rear axle assemblies at different angles. This difference in working angles results in a xe2x80x9cbroken backxe2x80x9d arrangement for the intermediate drive shaft disposed between the two yokes and subjects the universal joints coupling the intermediate drive shaft to the yokes to relatively large amounts of vibration and torsional stress. In a few prior art tandem axle assemblies, the output yoke and input yoke have the same working angles and, therefore, the intermediate drive shaft has a xe2x80x9cparallelxe2x80x9d arrangement in which the universal joints are subject to less vibration and torsional stress. Although the working angles are equal, however, the identical gear offsets result in working angles that are relatively large which results in relatively high levels of inertia during acceleration and deceleration of the vehicle.
The assignee of the present invention, Dana Corporation, has produced a tandem drive axle assembly called the DST 40 in which the output yoke and input yoke of the intermediate drive axle assembly exit and enter, respectively, the forward and rear axle assemblies at the same angle. As a result, the intermediate drive shaft has a xe2x80x9cparallelxe2x80x9d arrangement and the universal joints coupling the intermediate drive shaft to the yokes are not subject to the same degree of vibration and torsional stress. In order to achieve this parallel arrangement without producing large working angles, however, the DST 40 uses a spiral bevel gearset (with no offset between the pinion and ring gears) in the rear axle assembly. Therefore, although the DST 40 represents a significant improvement over conventional tandem axle assemblies by virtue of the parallel arrangement of the intermediate drive shaft, the DST 40 lacks the gear strength and low noise levels found in some prior art tandem axle assemblies.
There is thus a need for a tandem axle assembly that will minimize or eliminate one or more of the above-mentioned deficiencies.
The present invention provides a tandem axle assembly for a vehicle.
A tandem axle assembly in accordance with the present invention includes a forward axle assembly and a rear axle assembly. The forward axle assembly includes a forward ring gear and a forward pinion gear in mesh with the forward ring gear. The rear axle assembly includes a rear ring gear and a rear pinion gear in mesh with the rear ring gear. The forward and rear pinion gears rotate in response to an inter-axle differential. The forward pinion gear is offset from the forward ring gear by a first predetermined distance while the rear pinion gear is offset from the rear ring gear by a second predetermined distance. The second predetermined distance is greater than zero, but is less than the first predetermined distance.
A tandem axle assembly in accordance with the present invention has several advantages as compared to conventional tandem axle assemblies. The variation in offsets between the gears in the forward and rear axle assemblies of the tandem axle assembly produces working angles that are both equal and relatively small. The equal working angles enable a parallel arrangement for the intermediate drive shaft assembly thereby reducing vibration and torsional stress at the universal joints on either end of the drive shaft assembly. The small working angles reduce inertia during acceleration and deceleration of the vehicle. Finally, the use of an offset gear arrangement in both the forward and rear axle assemblies results in increased gear contact ratio in both assemblies thereby producing stronger and quieter gearing.
These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.